One example of Trinculo's cowardice is when he encounters Caliban, the deformed son of the witch Sycorax. Trinculo is terrified of Caliban, and he immediately tries to hide from him. He even goes so far as to disguise himself as a fish in order to avoid being eaten by Caliban.
Another example of Trinculo's foolishness is when he is given a drink by Stephano, the drunken butler of Prospero, the rightful Duke of Milan. Trinculo is so drunk that he doesn't realize that the drink is poisoned, and he ends up drinking it all. This leads to Trinculo becoming even more foolish and ridiculous.
Finally, Trinculo's drunken state is often used for comic relief. He is often seen stumbling around and slurring his words, and he frequently makes foolish remarks. This can be seen when he remarks after drinking from Stephano that "This is a brave god and bears celestial liquor. I will kneel to him," when it isn't actually a God; rather a mere bottle Stephano is holding.
All of these factors contribute to Trinculo's humor, and make him one of the most entertaining characters in The Tempest.